A telephone network is often used as an interface between a digital modem and an analog modem. Generally, a digital modem is a device that communicates digital data by using digital signals that replicate analog waveforms. An analog modem is a device that communicates digital data by encoding the data on analog waveforms.
FIG. 1 shows a typical telephone network 99 for interconnecting a digital modem 101 and an analog modem 102. The digital modem 101 is usually interconnected with a digital network 113 via digital connections 112a, 112b. For instance, the digital modem 101 may be interconnected to a digital network 113 in the form of a public switch telephone network (PSTN) via a Local Exchange Carrier (LEC) subscriber loop. The digital network 113 may comprise, among other things, a T1 carrier system, a basic rate or primary rate Integrated Services Digital Network (ISDN), a fiber optic cable network, a coaxial cable network, a satellite network, or even a wireless digital communications network. Communications over the digital network 113 are conducted in accordance with a pulse code modulation (PCM) scheme. Channel capacity through these digital facilities is typically between 56 and 64 kilobits per second (kb/s). Coding of the signals is also employed so that compression and a constant signal/distortion performance over a wide dynamic range is achieved for optimal transmission of voice signals.
A commonly used coding technique is a nonlinear mu-law coding. The linear amplitude L corresponding with each code word is encoded, or converted to, a mu-law code word by the following equation: EQU M=(L+33)*2.sup.8-N
where M is the mu-law magnitude (e.g., 4 bits), L is the linear amplitude (e.g., 14 bits), and N is the mu-law segment, or frame (e.g., 3 bits). The mu-law code word is decoded, or converted to, a linear code word as follows: EQU L={(2M+33)2.sup.N -33}
The digital network 113 is in turn interconnected with another LEC subscriber loop that includes a coder/decoder (codec) 106. The codec 106 is interconnected with the digital network 113 via digital connections 114a, 114b. The codec 106 is often situated at a telephone company office or along a street near the analog modem subscriber in a subscriber loop carrier (SLC) device. The codec 106 provides an interface between the digital network 113 and an analog telephone connection 118, sometimes referred to as a copper loop. For communications in the direction from the digital network 113 to the analog modem 102, the codec 106 includes a mu-to-linear digital-to-analog converter (DAC) 109. The converter 109 converts nonlinear mu-law levels to a linear analog signal.
For communications in the direction from the analog modem 102 to the digital network 113, the codec 106 includes a linear-to-mu analog-to-digital (ADC) converter 107. The converter 107 converts the linear analog signal to nonlinear mu-law code words.
A hybrid 103 is in communication with the DAC and ADC via respective low pass filters (LPFs) 111, 105. The hybrid 103 serves to separate the bidirectional analog signals from the analog telephone connection 118 into unidirectional transmit and receive analog signals sent to and received from the ADC 107 and the DAC 109, respectively.
Furthermore, the analog modem 102 is connected to the analog telephone connection 118 and communicates analog signals therewith. Thus, communications occur between the digital modem 101 and the analog modem 102 by way of the digital network 113 and the codec 106.
A method known as rob bit signaling (RBS) is oftentimes utilized in the digital network 113 to communicate on-hook/off-hook status between the modems 101, 102 and the digital network 113. RBS forces the least significant bit (LSB) of every nth frame, where n is typically 6 or 24, to a constant logic level, either logical 1 or 0. Unfortunately, RBS causes the block error rate of data transfers to increase and the peak error to increase from 0.5 LSB to 1.5 LSB, as is illustrated in FIG. 2.
More specifically, with reference to FIG. 2, logic states are decoded by a receive subsystem associated with the digital modem 101 in accordance with the decode step function indicated at reference numeral 122. The possible maximum error e resulting from the encoding/decoding process for any given signal level is 0.5 LSB. Further, when an RBS frame occurs, the LSB is driven to a predetermined logical state, either a mark (logical 1) or a space (logical 0). Therefore, the logic states are decoded as indicated by step functions 123 and 124 for the mark and space, respectively, as shown FIG. 2. The possible maximum error in both the 1-RBS frame and the 0-RBS frame resulting from the encoding/decoding process is 3*e, or 1.5 LSB. Almost needless to say, high speed modem performance is seriously degraded as a result of RBS.
Hence, there exists a need in the industry for systems and methods for coping with RBS and for increasing the speed of data transfers through the digital network that periodically robs a bit.